Photochromic compounds of various classes are known: indoline spiropyrans, indoline spirooxazines, benzo- and naphthopyrans, fulgides, diarylethenes, quinones, perimidinespirocyclohexadienones [V. Malatesta, Photodegradation of Organic Photochromes In Organic Photochromic and Thermochromic Compounds. Vol. 2 Eds. J. C. Crano and R. J. Guglielmetti. Kluwer Academic/Plenum Publishers, New York, 1991, 473 pp.], indolo[2,1-b][1,3]benzoxazines. Exposition the solutions of spiropyrans and spirooxazines to UV irradiation brakes C—O bond and coloured merocyanines form, which revert back to the initial state due to the thermal movement of molecules or after irradiation with visuable light during micro- and miliseconds. Known photochromic compounds—indolino spiropyrans, for example, are described in patents EP0411884A1, GB1270928A, JP2006249622A, U.S. Pat. Nos. 5,155,230, 5,241,075 and others. Dihydroindolines, similarly to indolino spiropyrans, after irradiation with UV are transfered to coloured form and revert back to the initial state thermaly, but the livetime of coloured form is longer comparing with indolinespiropyrans, and it is up to hundreds of milliseconds, minutes or even hours. [WO2012172093; R. Zemribo, J. Fotins, U. A. Holger Kubas et al., Photochromism of dihydroindolizines: part XIV. Synthesis and photophysical behavior of photochromic dihydroindolizine tripodal linkers toward anchoring sensitizers to semiconductor nanoparticles. J. Phys. Org. Chem. 2011, 24 173-184; T. B. Shrestha, J. Melin. Y. Liu, et al., New insights in the photochromic spiro-dihydroindolizine/betaine-system. Photochem. Photobiol. Sci., 2008, 7, 1449-1456]. Another known class of photochromic compounds—quinones also do not demonstrate very fast interchange of uncoloured-coloured forms ant it takes from few till hundreds of microseconds [N. P. Gritsan, L. S. Klimenko. J. Photochem. Photobiol. A: Chem. 1993, 70, 103-117].
Perimidinecyclohexadienones are characterized by extremely long lifetime of coloured form, which reaches few days due to Z-E configurational exchange, similarly to E-Z in merocyanines. Relatively recently discovered photochromic compounds benzo- and naphthopyrans are not sufficient photostable. Due to the action of UV irradiation pyrane ring is transformed to furane ring and initial compound loses photochromic properties. [C. D. Gabbutt, B. M. Heron, S. B. Kolla, et al. Ring contraction during the 6p-electrocyclisation of naphthopyran valence tautomers. Org Biomol Chem 2008; 6:3096-3104; U.S. Pat. No. 6,410,754B1]. Fulgides and fulgimides are characterized by electrocyclization reaction, occurring when they are exposed to UV irradiation, when coloured 1,3-cyclohexadiene fragment possessing compounds are formed. The latter are thermally stable and can be transferred into the initial state only after irradiation with visible light. Diarylethenes, which also are thermally stable, act similarly.
Some of the newest classes of photochromic compounds—indole[2,1-b][1,3]benzoxazines, have a thermally unstable colored form, but they are much faster than the indoline spiropyrans and lifetimes thereof are shorter than of indoline spiropyrans [U.S. Pat. No. 8,252,209]. A new class of photochromic compounds, 6-nitro-1,3,3′,4-tetrahydrospiro[chromen-2,2′-indoles], is described in Lithuanian patent LT6024 (WO2014/035244), which coloured form lifetime is only 22-41 ns.
When indoline spiropyrans are exposed to ultraviolet irradiation, breaking of pyrane ring occurrs and coloured merocyanine form appears [V. I. Minkin, Photoswitchable molecular systems based on spiropyrans and spirooxazines, 37-80 pp. In Molecular switches/edited by B. L. Feringa and W. R. Browne. Weinheim, Wiley-VCH, 2011, Vol. 1-2; Spiropyrans. R. C. Bertelson, 11-84 pp. In Organic Photochromic and Thermochromic Compounds, Vol. 1-2, Eds. J. C. Crano, R. J. Guglielmetti, Plenum Press. New York and London, 1999, Vol. 1-2.]. Thermal transformation of the formed merocyanine to spiropyran is retarded significally due to trans-cis reisomerization stage, which takes place when coloured form returns to initial spiropyran form, and this takes miliseconds.

Coloured form generation from indolino spiropyrans is relatively fast, but due to thermo- and photochemical degradation they stand less cycles and also are not resistant to air oxygen [V. Malatesta, Photodegradation of Organic Photochromes In Organic Photochromic and Thermochromic Compounds. Vol. 2 Eds. J. C. Crano and R. J. Guglielmetti. Kluwer Academic/Plenum Publishers, New York, 1991, 473 pp.]. Besides, spiropyrans in merocyanine form are easily hydrolysable [Stafforst, T., and Hilvert, D. Kinetic characterization of spiropyrans in aqueous media. Chem. Commun. 2009, 287-288].
A new class of photochromic compounds, namely 6-nitro-1,3,3′,4-tetrahydrospiro[chromen-2,2′-indoles], possessing carbon skeleton of indoline spiropyrans, where double bond in pyrane ring is hydrogenated, is described in the above-mentioned Lithuanian patent LT6024 (WO2014/035244).

After irradiation of such 1′,3,3′,4-tetrahydrospiro[chromen-2,2′-indoles] (A) with UV radiation a colored 4-nitrophenolate fragment (B) forms, which due to molecular thermal movement returns into the initial state during few tens of nanoseconds. This is because in the absence of double bond it is not necessary to isomerize from trans- to cis-isomeric forms. However compound A is very sensitive for alkaline medium.
Immediate substitution of phenolate at asymmetric carbon atom appears, and coloured compound C possessing 4-nitrophenol fragment forms, if any amount of hydroxide ions is present in the solution, and photochromic effect disappears.

Therefore, it is important to have photochromic compounds that respond quicker and will not lose their photochromic properties in alkaline solution.